1. Are you thinking of upgrading the Raspberry Pi Zero as you have done upgrades to the A and I think it would help with quite a few smaller projects?

I'm sure the folks at the RPF and RPT are always *thinking* about how to improve hardware. *Thinking* about it is the easy part. Any changes have to keep an eye on cost and feasibility. Upgrading the Pi0/Pi0W is very, very difficult due to both the cost constraints and available hardware. So while they might be *thinking* of upgrading those boards, if it ever happens it will be a long time in the future (and maybe never). The launch of the Pi3A+ is an entirely different matter. There is nothing actually *new* to the Pi3A+. It is, in practice, a "cut down" Pi3B+. In that sense, its existence is mostly a case the RPT/RPF deciding that enough of them will sell to justify the effort to get it on the market.

2. Is it possible to add sensors on the Pi like an Accelerometer, Compass, Gps etc. as this would be easy to do and helpful?

Read up on the "SenseHAT". It's a whole bunch of sensors, and an 8x8 matrix of RGB LEDs--that mounts on the GPIO pin block. You're not going to see those sensors integrated onto the main Pi PCB for a variety of reasons. A major reason is that it would add the cost of doing so to every such Pi sold...and most people don't need (or want) the sensor suite.

This brings up a couple of points... The first is your assertion that adding them to the board would be "easy". From this I surmise that you've never done PCB layout or device integration. Only the real pros can make it *look* easy. The second problem is the assertion that it would be helpful. That begs the question: Helpful to what fraction of Pi buyers?

3. Can you add parts to a Pi that enable a sim card to be inserted so that you can do Mobile Phone Projects?

If you can find something that can be connected through any of the existing interfaces--IIC, SPI, USB, ....--then you can add such a device. And people do add GSM modules to Pis. If you're asking if the RPT "could" add such a thing to the Pi PCB, the answer is yes, they could. However, if the question is *will* they do so, the answer is pretty much certainly an emphatic no.

The Pi is a true miracle of clever design and energetic negotiation. That such a device can be built--in quantity--and sold at the official prices is actually pretty astounding. Part of the reason it is even possible is that the very clever folks that designed the Pi figured what they *didn't* need to include to make a viable device. Things like power switches and real time clocks come immediately to mind. So while you is entirely free to ask for whatever features would tickle your particular fancy is all well and good, but they have to figure out if what you want would actually benefit the other 5 million buyers each year.